In the past, series compensation in transmission lines has been implemented by means of a thyristor-controlled or thyristor-connected series capacitor. However, these approaches are complex and costly because both a thyristor group and corresponding control devices are required. Similarly, past shunt compensators for transmission lines have also required the use of controlled thyristors, e.g., thyristor-controlled reactors.
Historically, earth fault current limiting was accomplished in electric power circuits by use of mechanically-adjusted reactors, e.g., Peterson coils. This approach required constant regulation of the reactor which is unwieldy because mechanical adjustment is required.